The present invention relates to a process for producing dielectric layers used as elements for constructing semiconductor devices.
It is well known that dielectric layers are essential elements for constructing semiconductors, e.g., gate insulating layers of field effect transistors and dielectric layers of capacitors, and cells comprising these elements, such as transistor-diode logic, transistor-transistor logic, read only memories, and dynamic random access memories.
Generally, the capacitance of capacitors is proportional to the relative dielectric constant of the dielectric material inserted between electrodes and inversely proportional to the distance between the electrodes. The trans conductance gm of field effect transistors is also proportional to the relative dielectric constant of the gate-insulating dielectric layer and inversely proportional to the thickness of the layer. Thus, it is desirable to produce dielectric layers of a material which has a high relative dielectric constant and also a high dielectric breakdown voltage.
Silicon dioxide (SiO.sub.2) layers have been mainly used as dielectric layers, the functions of which are set forth above, based on the fact that silicon dioxide has a relative dielectric constant of 3.8 and a dielectric breakdown voltage of 10 MV/cm. However, it is, practically, difficult to form a silicon dioxide layer having a thickness less than 10 nm without forming pin holes. This inevitably leads to an increased leakage current, and, consequently, a lower breakdown voltage.
Recently, in order to decrease the size of the elements and thus improve the integration of the elements, other dielectric materials having a relative dielectric constant higher than that of silicon dioxide have been used either tentatively or practically. Such dielectric materials are oxides of tantalum (Ta), titanium (Ti), niobium (Nb), hafnium (Hf), zirconium (Zr), yttrium (Y), and vanadium (V). For example, tantalum oxide exhibits a relative dielectric constant of from 20 to 28.
However, each of these dielectric oxides has inevitable disadvantages in that the breakdown voltage is low and the leakage of current is large. Of course, these disadvantages may be reduced by thickening the dielectric layer, which, however, leads to unfavorable lowering of the capacitance.
A process for increasing the breakdown voltage and decreasing the leakage of current of a Ta.sub.2 O.sub.5 /SiO.sub.2 double layer is proposed in which a silicon dioxide layer is first formed on a substrate and then a tantalum oxide (Ta.sub.2 O.sub.5) layer is formed thereon so as to form the Ta.sub.2 O.sub.5 /SiO.sub.2 double layer.
However, such a Ta.sub.2 O.sub.5 /SiO.sub.2 double structure exhibits a disadvantage in that the capacitance is low due to the thickness of the oxidized silicon, the thickness cannot be reduced owing to the occurrence of pin holes, as mentioned above. Furthermore, the electrical properties of the previously formed silicon dioxide layer deteriorate during the formation of tantalum oxide thereon, thereby lowering the breakdown voltage. In addition, such a double structure exhibits an unfavorable hysteresis with respect to the capacitance-voltage characteristic of a diode due to the abrupt heterojunction thereof.
A. G. Revesz et al of Comsat Laboratories teaches in the article "Film-Substrate Interaction in Si/Ta and Si/Ta.sub.2 O.sub.5 Structures" in the Journal of the Electrochemical Society, Vol. 123, No. 10, October 1976, that a tantalum film is deposited on a silicon substrate and oxidized. The resultant tantalum oxide film contains a significant amount of silicon, and the majority of the silicon atoms incorporated into the tantalum oxide film are concentrated near the interface while a small amount of the silicon atoms are concentrated at the outer surface of the tantalum oxide film. During oxidation, the thus-diffused silicon atoms are also oxidized by oxygen, which diffuses through the tantalum oxide. However, Revesz et al does not suggest the forming of an effective silicon oxide layer under the tantalum oxide layer so as to realize a dielectric layer adapted for use as elements of semiconductor devices.
B. E. Deal et al teaches in the article "General Relationship for the Thermal Oxidation of Silicon" in the Journal of Applied Physics, Vol. 36, No. 12, December 1965, that silicon is oxidized more quickly in wet oxygen than in dry oxygen. However, they do not suggest that oxygen can diffuse through a tantalum oxide layer.